The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head and substrate retainer of a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be a xe2x80x9cstandardxe2x80x9d pad in which the polishing pad surface is a durable, roughened surface, or a fixed- abrasive pad in which abrasive particles are held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slury, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate and by the resulting finish (e.g., absence of small-scale roughness) and flatness (e.g., absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In the planarization of semiconductor substrate wafers by CMP, it is known to use an annular retaining ring encompassing a wafer being polished for the purpose of preventing lateral movement of the wafer resulting from friction between the wafer and a moving polishing pad. See, e.g., U.S. Pat. No. 5,205,082 of Norm Shendon, et al., the disclosure of which is incorporated herein by reference.
A reoccurring problem in CMP is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency for the edge of the substrate to be polished at a different rate than the center of the substrate. The edge effect typically results in over-polishing (the removal of too much material from the substrate) of the perimeter portion of the substrate, e.g., the outermost five to ten millimeters, although the edge effect may also result in under-polishing. The over-polishing or under-polishing of the substrate perimeter reduces the overall flatness of the substrate, makes the edge of the substrate unsuitable for use in integrated circuits, and decreases the yield.
In one aspect, the invention provides a retainer for use in conjunction with a substrate polishing apparatus. The apparatus may have a polishing pad with a polishing surface for contacting a face of the substrate. The retainer has an inward facing retaining face for engaging and retaining the substrate against lateral movement during polishing of the substrate. The retaining face engages the substrate perimeter at more than substantially a single discrete circumferential location along the perimeter.
Various embodiments of the invention may include one or more of the following. The retaining face may engage the substrate perimeter at a least two discrete, spaced-apart, locations. The retaining face may engage the substrate perimeter at exactly two discrete, spaced-apart, locations. The retaining face may engage the substrate perimeter along at least a continuous circumferential zone of engagement. The circumferential zone of engagement may span at least 10xc2x0. The circumferential zone of engagement may span substantially the entire substrate perimeter. The retaining face may compressively engage the substrate perimeter at a plurality of circumferential locations along the perimeter.
The retaining face may be a continuous cylindrical inner surface of a continuous annular longitudinally-extending retainer portion. Such a retainer portion may have an opening for receiving the substrate at a lower end of the retainer portion and may have sufficient elasticity to accommodate the substrate while maintaining compressive engagement with the substrate. The retainer may be formed as an annular longitudinally-extending sleeve depending from a roof section of a retaining ring and separated from a body of the ring by an annular recess.
The retaining face may be a cylindrical inner surface of an annular longitudinally-extending sleeve portion. The retainer may further include an annular radially outwardly-extending flange, the sleeve portion depending from the flange, and the flange secured between a body of the retaining ring and a carrier body. Alternately, the retainer may include an annular radially-inwardly extending flange secured between a clamp and a membrane support structure.
The retaining face may be formed by an inner face of a band wrapped substantially entirely around the substrate and circumferentially adjustable to engage and release the substrate. The retainer may be elastomeric. The retainer may be formed as an annular lip depending from an substrate-backing membrane. The retainer may comprise a plurality of annular segments, each segment having a bottom face and a cylindrical inner face. The cylindrical inner faces of the segments may form a retaining face wherein the segments are selectively inwardly biasable so as to compressively engage the substrate perimeter. The retainer may comprise an inflatable annular bladder sandwiched between the segments and an inner face of a support structure so that inflation of the bladder biases the segments radially inward to engage the substrate perimeter.
By dispersing lateral contact forces between the retainer and substrate which otherwise would be concentrated at a single point of contact, the retainer may reduce localized distortions (e.g., vertical deflection of the substrate at the point of contact due to compression of the substrate by the retainer) in the substrate near its perimeter which might otherwise contribute to the xe2x80x9cedge effectxe2x80x9d.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.